"Practicing" on someone is out. If you swing, you're only practicing with people's parts. But if you're poly, you're practicing with people's hearts.
Your only choice is to talk to your husband about it. And honestly, if you can't do that, then you're not ready for polyamory. And you have to be ready for him to resist the idea. I think you know how to start that conversation--- you've already done it with us. "Hubby, I'm crushing hard on Friend. I want to be free to explore those feelings. I don't just want to swing with them. I want a relationship. I love you more than ever, but I could love him too. What do you think?" I would even ask him what he thought about Ms. Friend. The four of you are so close that he probably has a crush on her too. In which case, jackpot! I mean, let's get real: why else would they be moving in, if everybody didn't have feelings for everyone else, at least subconsciously.
...and then let the conversation go where it may. Be prepared for a "no." If he says no, then you have an opportunity at least to talk about why not. How does it make him feel? How does he think he would feel if you started a new relationship, etc., and you can negotiate a little bit.
But also be careful. My wife was in a similar situation with her previous husband. They were swingers, but she wanted more. When she did fall in love with one of her partners, her husband couldn't handle it. If your hubby gives you a firm "no," then, well, you have to understand that no means no.
I would not swing with this couple without that yes from your husband. You have feelings for him, and they live with you. Sex with them without clear boundaries would be a disaster. But you know that.